If Kha'zix mutated attack a lonely unit, and opponent reacts with ambush playing another unit, does he still trigger his effect?
Yes, Kha'Zix, Mutating Horror will still receive the +2 Might bonus and gain 2 XP.
Reasoning
According to Rule 359.3.f.4 and supported by RiftJudge FAQ #9849 and #10176, the condition "if an enemy unit is alone here" is evaluated at the specific moment the unit gains the "Attacker" or "Defender" combat designation.
- Trigger Condition: When Kha'Zix moves into the battlefield, he gains the "Attacker" designation. At this exact moment, the game checks the condition: Is an enemy unit alone at this battlefield?
- Condition Met: Since your opponent currently has only one unit at that location, the condition is satisfied. Kha'Zix's triggered ability is placed on the chain.
- Reaction: Your opponent plays a unit using Ambush. This adds their unit to the chain (or puts it into play if it were a standard entry, but Ambush specifically allows this as a [Reaction]).
- Resolution: Even if your opponent's play changes the state of the battlefield (adding a second unit), the state change happens after Kha'Zix's ability has already triggered and been placed on the chain. Because the condition was met at the time of triggering, the effect resolves successfully.
This is a specific interaction confirmed by multiple RiftJudge FAQs (e.g., #9849, #10176). The condition check is not re-evaluated when the ability resolves.